The present invention relates to the implantation into living tissue of mechanical implants made of an Ni-Ti alloy or a Ti-Nb alloy while utilizing the memory effect due to heating of the implants to a temperature above the temperature specific for the particular material.
It is known in medicine to use unchangeably rigid implants--such as, for example, nails, clamps and pins--with which tissue separations or breaks are connected together mechanically. In such cases, the separated or broken pieces of tissues, e.g. bones, are pressed against one another by means of external clamping devices and screwed plates, so that a pressure develops which enhances healing. Also known are prosthetic parts which serve as bond substitutes and which must either be cemented in place or must be screwed to the bone.
The art also knows materials under the name "memory alloys" (see, for example, German Auslegeschrift (Published Patent Application) No. 2,661,710 which, once they have been appropriately preshaped, are capable of performing mechanical work during a later heat treatment. This behavior of these alloy materials, which is based on a change in their structure, resides in that certain alloys, after plastic deformation will return to their original shape, i.e., the shape before the deformation after having been heated to above a certain temperature which is specific for the particular material involved. The temperature range in which a structure modification produced by a plastic deformation is spontaneously reversed is called the conversion temperature range. For an alloy consisting of 55 weight percent Ni and 45 weight percent Ti, this conversion range lies, for example, at about 60.degree. C. A change in the percentage composition or the addition of Fe, Co, Mn, Al, Au or zirconium produces a shift in the conversion temperature. Thus it is possible to actuate thermally controlled forces by suitable selection of the composition of a "memory alloy".
U.S. Pat. No. 3,786,806, issued Jan. 22nd, 1974, proposes to use plates of a memory alloy for surgical purposes. According to this patent, the plates of memory alloy are initially prestretched, are then fastened to the fractured bone pieces by means of screws and are then heated. Due to changes in their structure, the plates become shorter and if necessary, produce the pressure required to heal the fracture at that point. Upon completion of the healing process, the screws or similar fastening elements must be removed from the bone. There then exists the danger that the just healed bones receive tissue damage.